/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.ObjectArrays.checkElementNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.primitives.Ints;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.concurrent.LazyInit;
import com.google.j2objc.annotations.RetainedWith;
import java.io.Serializable;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.EnumSet;
import java.util.Iterator;
import java.util.Set;
import java.util.SortedSet;
import java.util.Spliterator;
import java.util.function.Consumer;
import java.util.stream.Collector;
import javax.annotation.Nullable;

/**
 * A {@link Set} whose contents will never change, with many other important properties detailed at
 * {@link ImmutableCollection}.
 *
 * @since 2.0
 */
@GwtCompatible(serializable = true, emulated = true)
@SuppressWarnings("serial") // we're overriding default serialization
public abstract class ImmutableSet<E> extends ImmutableCollection<E> implements Set<E> {
    static final int SPLITERATOR_CHARACTERISTICS =
            ImmutableCollection.SPLITERATOR_CHARACTERISTICS | Spliterator.DISTINCT;

    /**
     * Returns a {@code Collector} that accumulates the input elements into a new
     * {@code ImmutableSet}. Elements are added in encounter order; if the elements contain
     * duplicates (according to {@link Object#equals(Object)}), only the first duplicate in
     * encounter order will appear in the result.
     *
     * @since 21.0
     */
    @Beta
    public static <E> Collector<E, ?, ImmutableSet<E>> toImmutableSet() {
        return CollectCollectors.toImmutableSet();
    }

    /**
     * Returns the empty immutable set. Preferred over {@link Collections#emptySet} for code
     * consistency, and because the return type conveys the immutability guarantee.
     */
    @SuppressWarnings({"unchecked"}) // fully variant implementation (never actually produces any
                                     // Es)
    public static <E> ImmutableSet<E> of() {
        return (ImmutableSet<E>) RegularImmutableSet.EMPTY;
    }

    /**
     * Returns an immutable set containing {@code element}. Preferred over
     * {@link Collections#singleton} for code consistency, {@code null} rejection, and because the
     * return type conveys the immutability guarantee.
     */
    public static <E> ImmutableSet<E> of(E element) {
        return new SingletonImmutableSet<E>(element);
    }

    /**
     * Returns an immutable set containing the given elements, minus duplicates, in the order each
     * was first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all
     * except the first are ignored.
     */
    public static <E> ImmutableSet<E> of(E e1, E e2) {
        return construct(2, e1, e2);
    }

    /**
     * Returns an immutable set containing the given elements, minus duplicates, in the order each
     * was first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all
     * except the first are ignored.
     */
    public static <E> ImmutableSet<E> of(E e1, E e2, E e3) {
        return construct(3, e1, e2, e3);
    }

    /**
     * Returns an immutable set containing the given elements, minus duplicates, in the order each
     * was first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all
     * except the first are ignored.
     */
    public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4) {
        return construct(4, e1, e2, e3, e4);
    }

    /**
     * Returns an immutable set containing the given elements, minus duplicates, in the order each
     * was first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all
     * except the first are ignored.
     */
    public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4, E e5) {
        return construct(5, e1, e2, e3, e4, e5);
    }

    /**
     * Returns an immutable set containing the given elements, minus duplicates, in the order each
     * was first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all
     * except the first are ignored.
     *
     * @since 3.0 (source-compatible since 2.0)
     */
    @SafeVarargs // For Eclipse. For internal javac we have disabled this pointless type of warning.
    public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E... others) {
        final int paramCount = 6;
        Object[] elements = new Object[paramCount + others.length];
        elements[0] = e1;
        elements[1] = e2;
        elements[2] = e3;
        elements[3] = e4;
        elements[4] = e5;
        elements[5] = e6;
        System.arraycopy(others, 0, elements, paramCount, others.length);
        return construct(elements.length, elements);
    }

    /**
     * Constructs an {@code ImmutableSet} from the first {@code n} elements of the specified array.
     * If {@code k} is the size of the returned {@code ImmutableSet}, then the unique elements of
     * {@code elements} will be in the first {@code k} positions, and {@code elements[i] == null}
     * for {@code k <= i < n}.
     *
     * <p>
     * This may modify {@code elements}. Additionally, if {@code n == elements.length} and
     * {@code elements} contains no duplicates, {@code elements} may be used without copying in the
     * returned {@code ImmutableSet}, in which case it may no longer be modified.
     *
     * <p>
     * {@code elements} may contain only values of type {@code E}.
     *
     * @throws NullPointerException if any of the first {@code n} elements of {@code elements} is
     *         null
     */
    private static <E> ImmutableSet<E> construct(int n, Object... elements) {
        switch (n) {
            case 0:
                return of();
            case 1:
                @SuppressWarnings("unchecked") // safe; elements contains only E's
                E elem = (E) elements[0];
                return of(elem);
            default:
                // continue below to handle the general case
        }
        int tableSize = chooseTableSize(n);
        Object[] table = new Object[tableSize];
        int mask = tableSize - 1;
        int hashCode = 0;
        int uniques = 0;
        for (int i = 0; i < n; i++) {
            Object element = checkElementNotNull(elements[i], i);
            int hash = element.hashCode();
            for (int j = Hashing.smear(hash);; j++) {
                int index = j & mask;
                Object value = table[index];
                if (value == null) {
                    // Came to an empty slot. Put the element here.
                    elements[uniques++] = element;
                    table[index] = element;
                    hashCode += hash;
                    break;
                } else if (value.equals(element)) {
                    break;
                }
            }
        }
        Arrays.fill(elements, uniques, n, null);
        if (uniques == 1) {
            // There is only one element or elements are all duplicates
            @SuppressWarnings("unchecked") // we are careful to only pass in E
            E element = (E) elements[0];
            return new SingletonImmutableSet<E>(element, hashCode);
        } else if (tableSize != chooseTableSize(uniques)) {
            // Resize the table when the array includes too many duplicates.
            // when this happens, we have already made a copy
            return construct(uniques, elements);
        } else {
            Object[] uniqueElements = (uniques < elements.length) ? Arrays.copyOf(elements, uniques) : elements;
            return new RegularImmutableSet<E>(uniqueElements, hashCode, table, mask);
        }
    }

    // We use power-of-2 tables, and this is the highest int that's a power of 2
    static final int MAX_TABLE_SIZE = Ints.MAX_POWER_OF_TWO;

    // Represents how tightly we can pack things, as a maximum.
    private static final double DESIRED_LOAD_FACTOR = 0.7;

    // If the set has this many elements, it will "max out" the table size
    private static final int CUTOFF = (int) (MAX_TABLE_SIZE * DESIRED_LOAD_FACTOR);

    /**
     * Returns an array size suitable for the backing array of a hash table that uses open
     * addressing with linear probing in its implementation. The returned size is the smallest power
     * of two that can hold setSize elements with the desired load factor.
     *
     * <p>
     * Do not call this method with setSize less than 2.
     */
    @VisibleForTesting
    static int chooseTableSize(int setSize) {
        // Correct the size for open addressing to match desired load factor.
        if (setSize < CUTOFF) {
            // Round up to the next highest power of 2.
            int tableSize = Integer.highestOneBit(setSize - 1) << 1;
            while (tableSize * DESIRED_LOAD_FACTOR < setSize) {
                tableSize <<= 1;
            }
            return tableSize;
        }

        // The table can't be completely full or we'll get infinite reprobes
        checkArgument(setSize < MAX_TABLE_SIZE, "collection too large");
        return MAX_TABLE_SIZE;
    }

    /**
     * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
     * each appears first in the source collection.
     *
     * <p>
     * <b>Performance note:</b> This method will sometimes recognize that the actual copy operation
     * is unnecessary; for example, {@code copyOf(copyOf(anArrayList))} will copy the data only
     * once. This reduces the expense of habitually making defensive copies at API boundaries.
     * However, the precise conditions for skipping the copy operation are undefined.
     *
     * @throws NullPointerException if any of {@code elements} is null
     * @since 7.0 (source-compatible since 2.0)
     */
    public static <E> ImmutableSet<E> copyOf(Collection<? extends E> elements) {
        /*
         * TODO(lowasser): consider checking for ImmutableAsList here TODO(lowasser): consider
         * checking for Multiset here
         */
        // Don't refer to ImmutableSortedSet by name so it won't pull in all that code
        if (elements instanceof ImmutableSet && !(elements instanceof SortedSet)) {
            @SuppressWarnings("unchecked") // all supported methods are covariant
            ImmutableSet<E> set = (ImmutableSet<E>) elements;
            if (!set.isPartialView()) {
                return set;
            }
        } else if (elements instanceof EnumSet) {
            return copyOfEnumSet((EnumSet) elements);
        }
        Object[] array = elements.toArray();
        return construct(array.length, array);
    }

    /**
     * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
     * each appears first in the source iterable. This method iterates over {@code elements} only
     * once.
     *
     * <p>
     * <b>Performance note:</b> This method will sometimes recognize that the actual copy operation
     * is unnecessary; for example, {@code copyOf(copyOf(anArrayList))} should copy the data only
     * once. This reduces the expense of habitually making defensive copies at API boundaries.
     * However, the precise conditions for skipping the copy operation are undefined.
     *
     * @throws NullPointerException if any of {@code elements} is null
     */
    public static <E> ImmutableSet<E> copyOf(Iterable<? extends E> elements) {
        return (elements instanceof Collection) ? copyOf((Collection<? extends E>) elements)
                : copyOf(elements.iterator());
    }

    /**
     * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
     * each appears first in the source iterator.
     *
     * @throws NullPointerException if any of {@code elements} is null
     */
    public static <E> ImmutableSet<E> copyOf(Iterator<? extends E> elements) {
        // We special-case for 0 or 1 elements, but anything further is madness.
        if (!elements.hasNext()) {
            return of();
        }
        E first = elements.next();
        if (!elements.hasNext()) {
            return of(first);
        } else {
            return new ImmutableSet.Builder<E>().add(first).addAll(elements).build();
        }
    }

    /**
     * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
     * each appears first in the source array.
     *
     * @throws NullPointerException if any of {@code elements} is null
     * @since 3.0
     */
    public static <E> ImmutableSet<E> copyOf(E[] elements) {
        switch (elements.length) {
            case 0:
                return of();
            case 1:
                return of(elements[0]);
            default:
                return construct(elements.length, elements.clone());
        }
    }

    @SuppressWarnings("rawtypes") // necessary to compile against Java 8
    private static ImmutableSet copyOfEnumSet(EnumSet enumSet) {
        return ImmutableEnumSet.asImmutable(EnumSet.copyOf(enumSet));
    }

    ImmutableSet() {}

    /** Returns {@code true} if the {@code hashCode()} method runs quickly. */
    boolean isHashCodeFast() {
        return false;
    }

    @Override
    public boolean equals(@Nullable Object object) {
        if (object == this) {
            return true;
        } else if (object instanceof ImmutableSet && isHashCodeFast() && ((ImmutableSet<?>) object).isHashCodeFast()
                && hashCode() != object.hashCode()) {
            return false;
        }
        return Sets.equalsImpl(this, object);
    }

    @Override
    public int hashCode() {
        return Sets.hashCodeImpl(this);
    }

    // This declaration is needed to make Set.iterator() and
    // ImmutableCollection.iterator() consistent.
    @Override
    public abstract UnmodifiableIterator<E> iterator();

    @LazyInit
    @RetainedWith
    private transient ImmutableList<E> asList;

    @Override
    public ImmutableList<E> asList() {
        ImmutableList<E> result = asList;
        return (result == null) ? asList = createAsList() : result;
    }

    ImmutableList<E> createAsList() {
        return new RegularImmutableAsList<E>(this, toArray());
    }

    abstract static class Indexed<E> extends ImmutableSet<E> {
        abstract E get(int index);

        @Override
        public UnmodifiableIterator<E> iterator() {
            return asList().iterator();
        }

        @Override
        public Spliterator<E> spliterator() {
            return CollectSpliterators.indexed(size(), SPLITERATOR_CHARACTERISTICS, this::get);
        }

        @Override
        public void forEach(Consumer<? super E> consumer) {
            checkNotNull(consumer);
            int n = size();
            for (int i = 0; i < n; i++) {
                consumer.accept(get(i));
            }
        }

        @Override
        ImmutableList<E> createAsList() {
            return new ImmutableAsList<E>() {
                @Override
                public E get(int index) {
                    return Indexed.this.get(index);
                }

                @Override
                Indexed<E> delegateCollection() {
                    return Indexed.this;
                }
            };
        }
    }

    /*
     * This class is used to serialize all ImmutableSet instances, except for
     * ImmutableEnumSet/ImmutableSortedSet, regardless of implementation type. It captures their
     * "logical contents" and they are reconstructed using public static factories. This is
     * necessary to ensure that the existence of a particular implementation type is an
     * implementation detail.
     */
    private static class SerializedForm implements Serializable {
        final Object[] elements;

        SerializedForm(Object[] elements) {
            this.elements = elements;
        }

        Object readResolve() {
            return copyOf(elements);
        }

        private static final long serialVersionUID = 0;
    }

    @Override
    Object writeReplace() {
        return new SerializedForm(toArray());
    }

    /**
     * Returns a new builder. The generated builder is equivalent to the builder created by the
     * {@link Builder} constructor.
     */
    public static <E> Builder<E> builder() {
        return new Builder<E>();
    }

    /**
     * A builder for creating {@code ImmutableSet} instances. Example:
     * 
     * <pre>
     * {
     *     &#64;code
     *
     *     static final ImmutableSet<Color> GOOGLE_COLORS =
     *             ImmutableSet.<Color>builder().addAll(WEBSAFE_COLORS).add(new Color(0, 191, 255)).build();
     * }
     * </pre>
     *
     * <p>
     * Building does not change the state of the builder, so it is still possible to add more
     * elements and to build again.
     *
     * @since 2.0
     */
    public static class Builder<E> extends ImmutableCollection.ArrayBasedBuilder<E> {

        /**
         * Creates a new builder. The returned builder is equivalent to the builder generated by
         * {@link ImmutableSet#builder}.
         */
        public Builder() {
            this(DEFAULT_INITIAL_CAPACITY);
        }

        Builder(int capacity) {
            super(capacity);
        }

        /**
         * Adds {@code element} to the {@code ImmutableSet}. If the {@code
         * ImmutableSet} already contains {@code element}, then {@code add} has no effect (only the
         * previously added element is retained).
         *
         * @param element the element to add
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code element} is null
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> add(E element) {
            super.add(element);
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableSet}, ignoring duplicate
         * elements (only the first duplicate element is added).
         *
         * @param elements the elements to add
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code elements} is null or contains a null element
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> add(E... elements) {
            super.add(elements);
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableSet}, ignoring duplicate
         * elements (only the first duplicate element is added).
         *
         * @param elements the {@code Iterable} to add to the {@code ImmutableSet}
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code elements} is null or contains a null element
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> addAll(Iterable<? extends E> elements) {
            super.addAll(elements);
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableSet}, ignoring duplicate
         * elements (only the first duplicate element is added).
         *
         * @param elements the elements to add to the {@code ImmutableSet}
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code elements} is null or contains a null element
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> addAll(Iterator<? extends E> elements) {
            super.addAll(elements);
            return this;
        }

        @CanIgnoreReturnValue
        @Override
        Builder<E> combine(ArrayBasedBuilder<E> builder) {
            super.combine(builder);
            return this;
        }

        /**
         * Returns a newly-created {@code ImmutableSet} based on the contents of the
         * {@code Builder}.
         */
        @Override
        public ImmutableSet<E> build() {
            ImmutableSet<E> result = construct(size, contents);
            // construct has the side effect of deduping contents, so we update size
            // accordingly.
            size = result.size();
            return result;
        }
    }
}
